As is disclosed in JP2000-052817A, as a control method for an internal combustion engine capable of a lean burn operation, there is known a control method that compares the target torque that is calculated from an accelerator pedal opening degree or the like with a predetermined judgement value, selects a lean burn operation when the target torque is equal to or smaller than the judgement value, and selects a stoichiometric operation when the target torque is larger than the judgement value. Further, as is disclosed in JP11-022512A, there is also known a control method that switches the operation mode of an internal combustion engine from a lean burn operation to a stoichiometric operation when a difference that is equal to or larger than a predetermined value is generated between a target torque and an actual torque during a lean burn operation.
Further, there is also known a control method that quickly switches the operation mode of an internal combustion engine from a lean burn operation to a stoichiometric operation when an acceleration request by an operator is detected during the lean burn operation.